In the extraction of oil from sub-surface reservoirs, it is necessary to drill a borehole, or indeed several bore holes, in order to access the reservoir. Using current technology, the practice is to drill highly deviated wells, or indeed drill the well in the area of the reservoir horizontal to the access of a main borehole. This involves bending a drill string in the desired direction by adjusting a weight on a bit and by using an individual or combination of devices that adjust the fulcrum point between the bit and that point. Such methods result in a controlled bowing or bending of the drill string. Because the drill string is made of individual lengths of collars, drill bits or specialty tools held together by specially configured American Petroleum Institute (API) or industry proprietary connections, the stress at the connections is dependent upon the respective connection shoulder interface loading. By torquing up the respective drill string pieces relative to each other, the interface loading on the connection shoulder is roughly controlled. This torque measurement is influenced by the thread profile, the type of lubricant used, the calibration of torque readouts, etc. As a result the actual connection shoulder load is never exactly known. See FIG. 1.
During the controlled bowing or bending of the drill string that occurs during directional drilling, the shoulder interface can become unloaded sufficiently to allow the drilling mud, which is circulated at high pressure down through the drill string, to leak through the connection/shoulder into the annulus. Prolonged leakage leads to mud jet cutting, and possibly a washout. If the connection is sufficiently weakened, the drill strings can no longer transmit the torque or carry the weight, and a twistoff may occur. Similarly, if the drill string is rotating, the connection can fatigue due to cyclical stressing of the pin due to bending.
Known techniques for dealing with the above referenced problem associated with drilling in various directions currently relies on the basic principle that the thread profile of both box and pin are accurately machined to be defined API or proprietary thread profile and that thread dope (lubricant) is properly applied. The torque is applied via pulling systems (either hydraulic or winch activated) acting via tong levers which are secured to the respective pieces of the drill string that have to be tightened. The torquing system pulls the tongs in opposing directions radially to the axis of the drill string. The applied torque is calculated to have been applied when hydraulic or strain gauge type readouts, on the torquing system, reach precalculated values. Mechanical spinner systems work on a similar principle although the method of applying the torque may be distributed over a longer section of the drill string.
There are significant problems associated with existing methods of monitoring the torque in such connections. Because most oil-field threads that hold drill string components together are tapered, the ability to apply the correct shoulder load is directly proportional to the thread profile of the mating parts as well as the coefficient of friction. In either case, if the taper is too shallow or extreme, the shoulder load is either too low or too high. Similarly, if the thread lubricant results in too low a coefficient of friction, then there is a high shoulder load and a possible stretched pin. Alternatively, if the thread lubricant has too high a coefficient of friction, then the shoulder load will be too low.